It is known that many precursors of 5-fluorouracil (5-FU) are useful as antitumor agents, but in general their bioconversion efficiency is still insufficient for the treatment of patients suffering from tumors. Further they cause intestinal toxicities and immunosuppressive toxicities, which are their major and dose limiting toxicities, respectively.
U.S. Pat. No. 4,966,891 discloses precursors of 5-FU which are improved in the above mentioned aspect of bioconversion efficiency and toxicities. They are converted to 5'-deoxy-5-fluorocytidine (5'-DFCR) by acylamidases, to 5'-deoxy-5-fluorouridine (5'-DFUR) by cytidine deaminase, and then to 5-FU by pyrimidine nucleotide phosphorylase in vivo which is preferentially localized in the liver, small intestin and tumor tissues.